The present study proposes to assess for biobehavioral differences between 160 Black and White, adult, male, mild hypertensives and normatensives between the ages of 25-55 yrs. Plasma renin levels are used to categorize the hypertensives, and family history and resting blood pressures are used to divide the normotensives into high and low risk groups. Controls for age, SES, medical conditions, obesity, and the confounding effects of medication are instituted in a careful sampling and screening process. Measures of psychological (i.e. level of subjective distress, train anxiety, and anger-response style), physiological (i.e. blood pressure and heart rate), and biochemical (i.e. urine sodium and potassium levels, and plasma epinephrine, norepinephrine, and renin) reactivity to a set of 3 laboratory stressors: mental arithmetic, physical exercise and interpersonal conflict role play conditions are taken. Twenty four-hour ambulatory monitoring of blood pressure on a sub-sample of mild hypertensives is also proposed and compared with laboratory stress-reactivity patterns. Univariate, n-way repeated measures ANOVAs and restricted multivariate ANOVA (MANOVA) are proposed in assessing for between-group and within-group differences in biobehavioral reactivity to stress. It is expected that the results should provide meaningful information on the biological and psychological mechanisms differentiating Black and White responses to stress which may be implicated in the pattern of differential racial risk for essential hypertension.